diff --git a/src/browser/webapi/Document.zig b/src/browser/webapi/Document.zig
index 0ff835480..92abc1fcd 100644
--- a/src/browser/webapi/Document.zig
+++ b/src/browser/webapi/Document.zig
@@ -179,6 +179,21 @@ pub fn setLocation(self: *Document, url: [:0]const u8) !void {
     return frame.scheduleNavigation(url, .{ .reason = .script, .kind = .{ .push = null } }, .{ .script = frame });
 }
 
+// Approximation of quirks mode: an HTML document without a doctype is in
+// quirks mode. (Legacy doctypes that also trigger quirks are not detected.)
+pub fn isQuirksMode(self: *const Document) bool {
+    if (self._type != .html) {
+        return false;
+    }
+    var child = self._proto.firstChild();
+    while (child) |c| : (child = c.nextSibling()) {
+        if (c._type == .document_type) {
+            return false;
+        }
+    }
+    return true;
+}
+
 pub fn getCharset(self: *const Document) []const u8 {
     if (self._charset) |charset| {
         return charset;
@@ -1445,6 +1460,7 @@ pub const JsApi = struct {
         return frame._factory.node(Document{
             ._proto = undefined,
             ._type = .generic,
+            ._url = "about:blank",
             ._charset = "UTF-8",
         });
     }
@@ -1534,7 +1550,10 @@ pub const JsApi = struct {
     pub const characterSet = bridge.accessor(getCharacterSet, null, .{});
     pub const charset = bridge.accessor(getCharacterSet, null, .{});
     pub const inputEncoding = bridge.accessor(getCharacterSet, null, .{});
-    pub const compatMode = bridge.property("CSS1Compat", .{ .template = false });
+    pub const compatMode = bridge.accessor(getCompatMode, null, .{});
+    fn getCompatMode(self: *const Document) []const u8 {
+        return if (self.isQuirksMode()) "BackCompat" else "CSS1Compat";
+    }
 
     fn getCharacterSet(self: *const Document) []const u8 {
         return self.getCharset();
diff --git a/src/browser/webapi/Node.zig b/src/browser/webapi/Node.zig
index 59cc05669..d1ea31693 100644
--- a/src/browser/webapi/Node.zig
+++ b/src/browser/webapi/Node.zig
@@ -1398,7 +1398,12 @@ pub fn getElementsByClassName(self: *Node, class_name: []const u8, frame: *Frame
         try class_names.append(arena, try frame.dupeString(name));
     }
 
-    return collections.NodeLive(.class_name).init(self, class_names.items, frame);
+    const doc: ?*Document = if (self._type == .document) self._type.document else self.ownerDocument(frame);
+    const quirks = if (doc) |d| d.isQuirksMode() else false;
+    return collections.NodeLive(.class_name).init(self, .{
+        .names = class_names.items,
+        .case_insensitive = quirks,
+    }, frame);
 }
 
 /// Shared implementation of replaceChildren for Element, Document, and DocumentFragment.
diff --git a/src/browser/webapi/collections/node_live.zig b/src/browser/webapi/collections/node_live.zig
index 69e30d178..79b4bf87f 100644
--- a/src/browser/webapi/collections/node_live.zig
+++ b/src/browser/webapi/collections/node_live.zig
@@ -46,6 +46,13 @@ const Mode = enum {
     form,
 };
 
+pub const ClassNameFilter = struct {
+    names: [][]const u8,
+    // getElementsByClassName matches class names ASCII case-insensitively
+    // when the document is in quirks mode.
+    case_insensitive: bool = false,
+};
+
 pub const TagNameNsFilter = struct {
     namespace: ?Element.Namespace, // null means wildcard "*"
     local_name: String,
@@ -55,7 +62,7 @@ const Filters = union(Mode) {
     tag: Element.Tag,
     tag_name: String,
     tag_name_ns: TagNameNsFilter,
-    class_name: [][]const u8,
+    class_name: ClassNameFilter,
     name: []const u8,
     all_elements,
     child_elements,
@@ -261,14 +268,14 @@ pub fn NodeLive(comptime mode: Mode) type {
                     return self._filter.local_name.eqlSlice(el.getLocalName());
                 },
                 .class_name => {
-                    if (self._filter.len == 0) {
+                    if (self._filter.names.len == 0) {
                         return false;
                     }
 
                     const el = node.is(Element) orelse return false;
                     const class_attr = el.getAttributeSafe(comptime .wrap("class")) orelse return false;
-                    for (self._filter) |class_name| {
-                        if (!Selector.classAttributeContains(class_attr, class_name)) {
+                    for (self._filter.names) |class_name| {
+                        if (!Selector.classAttributeContainsCase(class_attr, class_name, self._filter.case_insensitive)) {
                             return false;
                         }
                     }
diff --git a/src/browser/webapi/selector/Selector.zig b/src/browser/webapi/selector/Selector.zig
index ec5c09532..f6bdae851 100644
--- a/src/browser/webapi/selector/Selector.zig
+++ b/src/browser/webapi/selector/Selector.zig
@@ -183,10 +183,18 @@ pub fn matchesUncached(arena: Allocator, el: *Node.Element, input: []const u8, f
 }
 
 pub fn classAttributeContains(class_attr: []const u8, class_name: []const u8) bool {
+    return classAttributeContainsCase(class_attr, class_name, false);
+}
+
+pub fn classAttributeContainsCase(class_attr: []const u8, class_name: []const u8, case_insensitive: bool) bool {
     if (class_name.len == 0 or class_name.len > class_attr.len) return false;
 
     var search = class_attr;
-    while (std.mem.indexOf(u8, search, class_name)) |pos| {
+    while (if (case_insensitive)
+        std.ascii.indexOfIgnoreCase(search, class_name)
+    else
+        std.mem.indexOf(u8, search, class_name)) |pos|
+    {
         const is_start = pos == 0 or isClassWhitespace(search[pos - 1]);
         const end = pos + class_name.len;
         const is_end = end == search.len or isClassWhitespace(search[end]);